The present invention relates to an apparatus for continuously heating and drying stone aggregate useful in the production of asphalt paving composition or the like, and which also has the ability to incinerate contaminated soil under environmentally acceptable conditions.
The effective cleanup and disposal of soils which are contaminated with petroleum products and other contaminants have recently become major environmental concerns. As one specific example of the problem, a large number of underground fuel tanks have been found to be leaking and releasing a variety of contaminants into the surrounding soil, and as these tanks are being removed and repaired pursuant to environmental regulations, the volume of contaminated soil has been growing. Many communities no longer permit such soil to be placed in landfills, and thus a need exits for the decontamination of the soils.
Copending and commonly owned application Ser. No. 07/565,945 discloses an apparatus for efficiently decontaminating soil, and which has several common components with a conventional asphalt production plant. This feature permits a conventional asphalt plant to be retrofitted by the addition of a relatively small number of additional components, so as to permit the apparatus to selectively produce asphalt, or to decontaminate soil.
Another known design for a soil decontamination apparatus which is utilized in combination with several of the components of a conventional asphalt plant, comprises a drum dryer for heating aggregate in the conventional manner, and which includes a discharge end which is enclosed by a breeching for conveying the heated material to an external conveyor. A rotary soil incinerator is mounted adjacent the discharge end of the drum dryer, and the incinerator includes a discharge end which communicates with the breeching so that the incinerated soil may be discharged into the breeching and mixed with the heated aggregate. This system has proven to be very satisfactory, but problems remain by reason of the fact that the incineration of the soil produces unacceptably high levels of harmful NOx gases, which exit with the exhaust of the drum dryer.
Copending and commonly owned application entitled "Aggregate Dryer For Use With Asphalt Plant Having Reduced NOx Emissions", Ser. No. 07/633,334, filed Dec. 27, 1990, discloses a rotary dryer for heating aggregate in an asphalt plant, and which includes a recirculation system for recirculating gases into the downstream end of the gas burner for the purpose of reducing NOx emissions. However, this prior system is not seen to be applicable to the problem associated with the emission of NOx gases in an apparatus comprising a drum dryer and soil incinerator of the type described above.
It is accordingly an object of the present invention to provide an apparatus for decontaminating soil which may be used in conjunction with a drum dryer of an otherwise conventional asphalt plant, and which effectively reduces the release of harmful NOx gases.
It is a further object of the present invention to provide an apparatus of the described type wherein the incinerated soil may be selectively added to the heated and dried aggregate as it is discharged from the drum dryer, or directed to a separate external conveyor for stockpiling or other use.
Another object of the present invention is to provide an apparatus of the described type which includes provision for incinerating any volatile hydrocarbons contained in air scavenged from various locations of the asphalt plant.